1. Technical Field
The present invention relates to a high-strength member for vehicles, the high-strength member including a high-strength steel sheet having high strength and ability to absorb impact energy to high degrees.
2. Background Art
Recently, requirements for crash safety in vehicles have been increased. For example, as a safety measure for frontal collisions, a method in which the front frame is deformed to absorb energy, whereas deformation of the interior passenger space is minimized by high deformation resistance to maintain the space, is regarded as effective. In this method, the amount of energy absorption of the front frame is proportional to a product of the deformation resistance and deformation stroke. If a similar level of energy absorption were accomplished by a shorter deformation stroke, various benefits, such as improvement in motion performance and weight reduction of the vehicle body by a shortened front overhang, can be obtained. Therefore, recently, strengths of materials used in the front frame (generally, steel sheets) have been increased.
In order to increase the strength of the steel sheet for a front frame, since yield point is inevitably increased in the case in which the strength of the steel sheet is increased, it is therefore necessary to consider that initial reactive force, that is, reactive force at the moment of collision of the vehicle body, can also be greatly increased. Therefore, it is necessary that the initial reactive force be extremely minimized and that the absorption energy during deformation be sufficiently maintained.
Furthermore, under circumstances in which a part, such as a front frame, is compressed along its longitudinal direction, in the case in which strength of the steel sheet in increased, a buckled shape generally becomes unstable, and a mode of deformation changes from a stable shape buckled in the shape of a accordion to a bent condition. Obviously, since absorption efficiency of impact energy is decreased in the bent condition, improvement of absorption energy by strengthening the material cannot be anticipated in that case. As a reason that buckling becomes unstable by strengthening the steel sheet, it is mainly thought that work-hardening ability is decreased by strengthening the steel sheet material. That is, in the case in which degree of work-hardening of the material is high when the member is buckled once in the axial direction, deformation is dispersed not only to the initial buckled portion, but also to its surrounding region, another region is subsequently buckled, and as a result, a buckled form having an accordioned shape is obtained. However, in the case in which the degree of work-hardening of the material is low, deformation is concentrated at the initial buckled portion, and as a result, a bent form is obtained. Since work-hardening ability is generally decreased by strengthening the steel sheet, it has been inevitable that the buckling would be destabilized.
To solve such a problem, it is effective for the shape of the part to be formed so as to buckle stably. However, there is a limitation from the viewpoints of layout and design in an engine compartment, and a part having a desirable shape is not always used. Therefore, if a property of the material itself can be optimized to accomplish this purpose, it becomes possible for energy to be absorbed while strengthening the material without problems. Practically, if a steel sheet having high strength, low yield strength, and high work-hardening ability is used, increase of initial reactive force can be suppressed, buckling can be stabilized, and impact energy can be efficiently absorbed.
Furthermore, in a component part for an vehicle body such as the front frame, the part rarely consists of only one kind of steel sheet; most of the parts generally consist of at least two kinds of steel sheets or even more than two kinds. This is because comprehensive optimization has been promoted from the viewpoint of bonding properties, weight reduction, production cost and the like. Therefore, bonding of different materials is basically performed in the part for an vehicle body, and a requirement in which a bonding region of the materials of the parts does not break away when there is a collision is often specified as one of the requirements for crash safety in vehicles. A reason for avoiding the breaking is that desired absorption efficiency of impact energy is decreased by the breaking of the component materials. Occurrence or non-occurrence of breaking depends on bonding conditions, and the factor that is the most influential in selected bonding conditions is the kind of steel sheet. Therefore, in the case in which a component part for an vehicle body consisting of two or more kinds of steel sheet is designed, it is an important factor to select the kind of steel sheet.
As a steel sheet for a part for an vehicle body having superior collision properties, a steel sheet in which an austenite structure, possibly generating martensite by work-induced transformation and in which a steel sheet having a work hardening coefficient of not less than 0.6 is used, is disclosed (See Reference 1). In addition, a process for production of high-strength steel sheets having superior ductility, in which a steel having C: 0.1 to 0.45% and Si: 0.5 to 1.8% is hot-rolled, cold-rolled and annealed in prescribed conditions to obtain a steel having a tension strength of 82 to 113 kgf/mm2 and having a product of tension strength and extension of not less than 2500 kgf/mm2·% is disclosed (See Reference 2). Furthermore, a process for production for a high-strength steel sheet having high ductility, in which a composition having C: 0.1 to 0.4 wt % and suppressing Si is increased with a Mn amount and annealed twice in prescribed conditions to obtain a steel having a tension strength of 811 to 1240 MPa and having a product of tension strength and extension of not less than 28000 MPa·% is disclosed (See Reference 3).
Each reference mentioned above is a Japanese Unexamined Patent Application Publication, numbered as follows.    Reference 1: No. 2001-130444    Reference 2: No. Showa 62 (1987)-182225    Reference 3: No. Heisei 07 (1995)-188834.    Reference 4: No. 2007-321207